1. Field of the Invention
The present application relates to a thermally efficient compressed natural gas injector having improved performance.
2. Description of the Related Art
Compressed natural gas (hereinafter sometimes referred to as "CNG") is becoming a common automotive fuel for commercial fleet vehicles and residential customers. In vehicles, the CNG is delivered to the engine in precise amounts through gas injectors, hereinafter referred to as "CNG injectors". The CNG injector is required to deliver a precise amount of fuel per injection pulse and maintain this accuracy over the life of the injector. In order to maintain this level of performance for a CNG injector, certain strategies are required to help reduce the effects of contaminants in the fuel.
Compressed natural gas is delivered throughout the country in a pipeline system and is mainly used for commercial and residential heating. While the heating systems can tolerate varying levels of quality and contaminants in the CNG, the tolerance levels in automotive gas injectors is significantly lower.
These contaminants, which have been acceptable for many years in CNG used for heating, affect the performance of the injectors to varying levels and will need to be considered in future CNG injector designs. Some of the contaminants found in CNG are small solid particles, water, and compressor oil. Each of these contaminants needs to be addressed in the injector design for the performance to be maintained over the life of the injector.
The contaminants can enter the pipeline from several sources. Repair, maintenance and new construction to the pipeline system can introduce many foreign particles into the fuel. Water, dust, humidity and dirt can be introduced in small quantities with ease during any of these operations. Oxides of many of the metal types found in the pipeline can also be introduced into the system. In addition, faulty compressors can introduce vaporized compressor oils which blow by the seals of the compressor and enter into the gas. Even refueling can force contaminants on either of the refueling fittings into the storage cylinder. Many of these contaminants are likely to reach vital fuel system components and alter the performance characteristics over the life of the vehicle.
In general, fuel injectors require extremely tight tolerances on many of the internal components to accurately meter the fuel. For CNG injectors to remain contaminant tolerant, the guide and impact surfaces for the armature needle assembly require certain specifically unique characteristics.
In addition to fuel continuation problems using CNG the fuel injectors inherently present additional problems. For example, problems inherent to generation of heat in the solenoid coil are particularly aggravated in fuel injectors using CNG as will be explained hereinbelow.
The CNG (Compressed Natural Gas) injector is required to open and close very quickly to promote efficient fuel consumption. In order to accomplish this objective effectively the magnetic circuit utilized to open the value needle must produce a magnetic field--or flux--relatively quickly across the working gap between the fuel inlet connector and the armature. The CNG injector has a magnetic circuit consisting of an inlet connector, armature, valve body shell, housing and a coil. When energized, the coil produces a magnetic field which is conducted through the magnetic circuit. The flux is conducted through the components and creates an attractive force at the working gap, which force causes upward movement of the armature, with consequent upward movement of the valve needle to open the injector valve.
The CNG injector is required to open and close very quickly. This quick opening creates a relatively severe impact between the armature and the inlet connector. In the CNG injector, the factors which effect impact velocity between the armature and inlet connector are more severe then in a gasoline injector. Compared to a gasoline injector, the CNG injector has two to three times the lift, less spring preload and similar force required to open the injector. The difference is then exaggerated by the lower velocity (CNG) fluid then gasoline.
A CNG injector requires a much higher flow rate and area to obtain the same amount of energy flow through the injector in a given pulse. This is caused by the lower density of the gaseous CNG when compared to standard gasoline. This requires that the lift for a CNG injector be much greater than it is for a gasoline injector.
The increased lift creates several problems. First, the increased lift substantially reduces the magnetic force available to open the injector. Second, the velocities created because of the longer flight times can be higher, creating higher impact momentum. The reduction in magnetic force also creates another problem. This reduction in force requires the use of a lighter spring preload than in a standard gasoline injector.
In addition, with CNG, greater volumes of fuel are made to pass through the injector with increased demands on the solenoid coil and with of excessive heat which adversely affects the temperature and performance of the solenoid. Also, gaseous fuels have a lower specific heat than liquid fuels and thereby tend to conduct less heat away from the solenoid coil. We have invented a fuel injector suitable for use with compressed natural gas which conducts heat from the magnetic solenoid coil to the adjacent components so as to improve performance.